It's official: the kilogram is getting heavier

The kilogram has been getting heavier, meaning we are all getting slightly
lighter, experts say.

The standard unit for the kilogram weight – known as the International Prototype Kilogram (IPK) – is likely to be heavier now than when it was first introduced in 1875, scientists believe.

This is down to tiny particles of pollution attaching themselves to the platinum-based models.

But the kilo is only likely to have gained tens of micrograms over the 137 years, putting any pounds put on at Christmas into perspective.

The original IPK is a matchbox sized piece of platinum stored in the International Bureau of Weights and Measures in Paris. Forty official replicas were made in 1884 and distributed around the world in order to standardise mass.

But experts from the University of Newcastle say that industrialisation and modern living have taken their toll and contaminants have built up on the surfaces.

Each may have gained slightly different weights, effecting standardisation, they warn.

Professor Peter Cumpson and Dr Naoko Sano used a Theta-probe X-ray Photoelectron Spectroscopy (XPS) machine – the only one of its kind in the world – to analyse surfaces similar to the standard kilogram.

They assessed the likely build-up of hydrocarbons and also pondered how best to remove them, reports the Journal of Metrologia.

They found that a 'suntan' of UV and ozone was effective in returning them to their original mass.

Prof Cumpson said: "Statute decrees the IPK is the kilogram.

"It doesn't really matter what it weighs as long as we are all working to the same exact standard, the problem is there are slight differences.

"Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.

"We're only talking about a very small change – less than 100 micrograms – so, unfortunately, we can't all take a couple of kilograms off our weight and pretend the Christmas overindulgence never happened.

"But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge.

"There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for.

"What we have done at Newcastle is effectively give these surfaces a suntan. By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight."

Work is under way in several National Measurement Institutes to find an alternative to the IPK – a standardised value for the kilogram that is not based on a matchbox- sized piece of metal.

But until then, the prototype kilograms are what the world relies on for its mass scale.

"If the kilogram does put on weight then it's imperative that we understand exactly how the IPK is changing," said Professor Cumpson.

He added that it was the cutting edge technology which had allowed the team to make their calculations.

He said: "The Theta probe allows us to look at the composition of very thin layers by measuring the angle at which the electrons emerge from it.

"Rather like an MRI scanner, it takes a cross section of the material but at an atomic level."

The second part of the machine is the Argon cluster ion gun – which fires charged 'droplets', each containing about a thousand Argon atoms – and it is this which makes the Newcastle machine unique.

"The Argon cluster ion gun allows us to analyse organic materials without damaging the inorganic surface, in this case the platinum alloy," said Professor Cumpson.